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From "Dawid Weiss (Updated) (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (LUCENE-3867) RamUsageEstimator.NUM_BYTES_ARRAY_HEADER and other constants are incorrect
Date Fri, 23 Mar 2012 10:41:36 GMT

     [ https://issues.apache.org/jira/browse/LUCENE-3867?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Dawid Weiss updated LUCENE-3867:
--------------------------------

    Attachment: LUCENE-3867.patch

Don't be scared by the size of this patch -- it contains a lot of generated code in WildClasses.

Improvements:
- size estimation is not recursive (which led to stack overflows quite easily on more complex
object graphs),
- decreased memory consumption by using a custom impl. of an identity object set. 
- added a cache of resolved class information (ref. fields, shallow size).
- removed the optimization of counting only subclass field offsets because fields can be packed
(J9).
- added more verbose information about unsupported JVM features. J9 doesn't have the MX bean
for example (and does dump this).

The above changes also speed up the entire processing.
                
> RamUsageEstimator.NUM_BYTES_ARRAY_HEADER and other constants are incorrect
> --------------------------------------------------------------------------
>
>                 Key: LUCENE-3867
>                 URL: https://issues.apache.org/jira/browse/LUCENE-3867
>             Project: Lucene - Java
>          Issue Type: Bug
>          Components: core/index
>            Reporter: Shai Erera
>            Assignee: Uwe Schindler
>            Priority: Trivial
>             Fix For: 3.6, 4.0
>
>         Attachments: LUCENE-3867-3.x.patch, LUCENE-3867-compressedOops.patch, LUCENE-3867.patch,
LUCENE-3867.patch, LUCENE-3867.patch, LUCENE-3867.patch, LUCENE-3867.patch, LUCENE-3867.patch,
LUCENE-3867.patch, LUCENE-3867.patch, LUCENE-3867.patch, LUCENE-3867.patch, LUCENE-3867.patch,
LUCENE-3867.patch, LUCENE-3867.patch, LUCENE-3867.patch, LUCENE-3867.patch, LUCENE-3867.patch,
LUCENE-3867.patch, LUCENE-3867.patch
>
>
> RamUsageEstimator.NUM_BYTES_ARRAY_HEADER is computed like that: NUM_BYTES_OBJECT_HEADER
+ NUM_BYTES_INT + NUM_BYTES_OBJECT_REF. The NUM_BYTES_OBJECT_REF part should not be included,
at least not according to this page: http://www.javamex.com/tutorials/memory/array_memory_usage.shtml
> {quote}
> A single-dimension array is a single object. As expected, the array has the usual object
header. However, this object head is 12 bytes to accommodate a four-byte array length. Then
comes the actual array data which, as you might expect, consists of the number of elements
multiplied by the number of bytes required for one element, depending on its type. The memory
usage for one element is 4 bytes for an object reference ...
> {quote}
> While on it, I wrote a sizeOf(String) impl, and I wonder how do people feel about including
such helper methods in RUE, as static, stateless, methods? It's not perfect, there's some
room for improvement I'm sure, here it is:
> {code}
> 	/**
> 	 * Computes the approximate size of a String object. Note that if this object
> 	 * is also referenced by another object, you should add
> 	 * {@link RamUsageEstimator#NUM_BYTES_OBJECT_REF} to the result of this
> 	 * method.
> 	 */
> 	public static int sizeOf(String str) {
> 		return 2 * str.length() + 6 // chars + additional safeness for arrays alignment
> 				+ 3 * RamUsageEstimator.NUM_BYTES_INT // String maintains 3 integers
> 				+ RamUsageEstimator.NUM_BYTES_ARRAY_HEADER // char[] array
> 				+ RamUsageEstimator.NUM_BYTES_OBJECT_HEADER; // String object
> 	}
> {code}
> If people are not against it, I'd like to also add sizeOf(int[] / byte[] / long[] / double[]
... and String[]).

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